Abstract

We exploit the superb resolution of the new HST/ACS mosaic image of M51 to select a large sample of
young (<1 Gyr) star clusters in the spiral disk, based on their sizes. The image covers the entire spiral
disk in B, V, I and Hα, at a resolution of 2 pc per pixel. The surface density distribution of
4357 resolved clusters shows that the clusters are more correlated with clouds than with stars, and we find
a hint of enhanced cluster formation at the corotation radius. The radius distribution of a sample of 769
clusters with more accurate radii suggests that young star clusters have a preferred effective radius of
~3 pc, which is similar to the preferred radius of the much older GCs. However, in contrast to the
GCs, the young clusters in M51 do not show a relation between radius and galactocentric distance. This
means that the clusters did not form in tidal equilibrium with their host galaxy, nor that their radius
is related to the ambient pressure.